In late years, mobile communication devices, particularly mobile phones, have been rapidly widespread and service areas thereof cover most of Japan. Nonetheless, there still are areas, such as inside houses and buildings and at bad locations behind mountains or buildings, providing no service or unstable communication as unable to receive radio waves, or due to weak radio waves, from existing usual base stations (that is, radio base stations connected to the mobile communication networks via dedicated lines (referred to as macrocells, hereinafter). Moreover, in highly populated areas, such as urban areas, since a single macrocell accepts numerous mobile phones, it generates areas having troubles for connection despite sufficient radio wave strength and areas with slow communication speed.
In order to cover such areas with bad communication conditions, small base stations have been developed to cover small areas (approximately a few meters to a few tens of meters in radius) with small power output. Among such small base stations, one connected to the mobile communication network via a public communication line (a broadband network such as ADSL) widespread for domestic use is particularly referred to as a “femtocell” (see Non-Patent Document 1).
The femtocell can use the public line as a connection line to the mobile communication network and can be installed by users. Accordingly, a user can improve a communication area by installing the femtocell in a place where the user wishes to use his/her mobile phone, even if it is inside a house or behind buildings where the radio wave environment is not good. In addition, the femtocell allows for establishment of a spot communication area in a macrocell service area in an urban area. Thus, if a user wishes to improve the communication speed, he/she can install the femtocell and enjoy the same services (voice call, email, SMS (Short Message Service), Internet browsing and the like) as a conventional macrocell at an even faster communication speed. This offers a great advantage for carriers as well. That is, since the users voluntarily use the femtocells and the public line, it saves resources (such as radio bands and fixed networks) of the macrocell, which are supposed to be consumed originally. As a result, the carriers can improve the service area of the mobile phones at low cost.